The present invention relates to a fuel injector.
A fuel injector is described in German Published Patent Application No. 195 00 706.
The device for metering of liquids and gases, in particular in fuel injectors in internal combustion engines as described in German Published Patent Application No. 195 00 706, has a hydraulic travel amplifier for converting the actuator travel of a piezoelectric actuator into an enlarged lift of a valve needle. In order to integrate the travel amplifier into the valve housing in a small volume, the lifting cylinder of the path amplifier is provided with an end section having a reduced diameter that projects into a recess in the working piston of the path amplifier. A disk spring inserted into the amplifier chamber bordered by the piston applies the working piston to the actuator, and a helical compression spring arranged concentrically with the end section in the recess presses the lifting cylinder against the valve needle.
Effects of wear, manufacturing tolerances, and changes in temperature on the actuator travel are compensated by providing a fluid-filled hollow cylindrical throttle gap on the guide surfaces of the amplifier pistons between the amplifier pistons and between the amplifier pistons and the inside wall of the valve housing, through which the amplifier chamber communicates with a fluid-filled low-pressure space. The volume defined by the amplifier chamber, the throttle gaps and the low-pressure space is closed.
One disadvantage of the lifting device known from German Published Patent Application No. 195 00 706 is mainly the complicated design and the length of the valve. Furthermore, there is a high cavitation tendency in the throttle gaps due to the large displacement volumes.
German Patent No. 197 02 066 describes a fuel injector in which the change in length of the actuator is compensated by an appropriate combination of materials. The fuel injector known from this publication has an actuator which is guided in a valve housing under a spring pre-tension and works together with an actuating part composed of an actuating body and a head part; the head part rests on the piezoelectric actuator and the actuating body passes through an internal recess in the actuator. The actuating body is mechanically linked to a valve needle. In operation of the actuator, the valve needle is operated against the spray direction.
The actuator and the actuating body have at least approximately the same length and are made of a ceramic material or a material which resembles ceramic in terms of thermal expansion. The result of using materials such as INVAR in the same lengths and having the same coefficients of thermal expansion is that the actuator and the actuating body expand uniformly under the influence of heat.
The main disadvantage of this arrangement is its limited suitability for use in systems subject to great temperature fluctuations. The arrangement known from German Patent No. 197 02 066 does not achieve the present object due to the nonlinear characteristic of the thermal expansion coefficient of piezoceramic materials with varying temperature. Another disadvantage is the great manufacturing complexity which is associated with a relatively high cost due in particular to the choice of materials (e.g., INVAR).
The fuel injector according to the present invention has the advantage over the related art that temperature compensation does not depend on the coefficients of thermal expansion of the piezoceramic material. Thermal expansion is compensated by a hermetically sealed lifting device. This guarantees reliable and accurate operation of the fuel injector. The lifting device may be prefabricated as an independent subassembly, optionally in one unit together with the valve needle, and filled with a suitable hydraulic medium before being inserted into the fuel injector.
Due to the hermetic seal of the lifting device, leakage losses and an influx of fuel into the lifting device are prevented.
The design of the flexible sections as corrugated pipes is simple to manufacture and is thus inexpensive. Corrugated pipes are also advantageous for the equalizing volumes, because expansion of the hydraulic medium due to temperature is compensated by the flexibility of the corrugated pipe.
The guidance of the lifting cylinders in one another or in the stationary section of the housing without projections ensures a low tendency to skewing and thus ensures trouble-free operation even at high operating speeds.
Due to the large-dimension bores in the lifting cylinder in comparison with the leakage gap, there is little tendency to cavitation due to flow and turbulence.